Dementias of the Alzheimer type include, but are not limited to Alzheimer's disease, Parkinson's disease dementia, and related maladies in humans involving cognitive and behavioral dysfunction such as Lewy body dementia. Most are chronic neurodegenerative disorders of the human central nervous system (CNS) characterized by progressive cognitive impairment, a variety of neurobehavioral and/or neuropsychiatric disturbances, and restrictions in activities of daily living.
Alzheimer's disease is the most common form of dementia. Prevalence studies indicated that in 2000 there were about 25 million persons with Alzheimer's disease worldwide and this number is expected to increase to 114 million by 2050 unless an effective preventive or neuroprotective therapy emerges. Onset usually occurs in those over age 65. Clinical signs include progressive cognitive loss and other associated neurobehavioral disabilities together with a declining capability of performing the activities of daily living.
The basic cause of sporadic Alzheimer's disease is not known, probably because the disease is heterogeneous and involves age-related changes together with a complex interaction of genetic and environmental risk factors. Current hypotheses advanced to explain the pathophysiology of Alzheimer's disease center on the putative deleterious effects of the two misfolded and aggregated proteins, extracellular beta amyloid and intracellular tau. Presumably, as a consequence of the selective neurodegenerative process, the synthesis of the neurotransmitter acetylcholine declines. This reduction undoubtedly interferes with normal synaptic transmission in brain. Drugs that act to correct the acetylcholine deficiency thus constitute the mainstay of current therapy.
Dementias of the Alzheimer type also include cognitive impairments associated with Parkinson's disease. One example is Parkinson's disease dementia, also a chronic progressive CNS degenerative disorder with relatively late life onset. Parkinson's disease itself primarily affects motor function. But secondary symptoms include cognitive deterioration, especially deficits in executive function.
Another dementia of the Alzheimer type that is commonly linked to Parkinson's disease is known as dementia with Lewy bodies or Lewy body dementia. Although Lewy body dementia is now generally regarded as a separate disease, differentiation from Alzheimer's disease and from Parkinson's disease dementia may be clinically challenging. Lewy body dementia thus tends to be under-diagnosed or misdiagnosed as Alzheimer's disease or Parkinson's disease dementia. The clinical presentation of Lewy body dementia is typically one of cortical and subcortical cognitive impairment, with visuospatial and executive dysfunction more pronounced than in Alzheimer's disease. Core clinical features of Lewy body dementia, in addition to parkinsonism, are cognitive decline plus fluctuations in attention and recurrent visual hallucinations.
Both Parkinson's disease dementia and Lewy body dementia are characterized neuropathologically by the presence of cortical Lewy body pathology and synuclein protein deposition. Genetic factors appear to play a role in pathogenesis. Not surprisingly, the pathology of Parkinson's disease dementia and Lewy body dementia is heterogeneous and overlapping, often intermixed with changes of the Alzheimer and vascular types. A reduction in brain acetylcholine-mediated neurotransmission has been linked to the primary clinical abnormalities found in both these disorders and drugs acting to stimulate cholinergic transmission now constitute the main approach to therapy.
In addition to the aforementioned disorders, the off label administration of drugs that augment CNS cholinergic transmission for various other cognitive disorders is widespread. Some of this use involves cognitive disorders for which relatively little clear evidence of cholinergic dysfunction currently exists. Nevertheless, an increasing number of clinical studies now support a rational extension of AChEI treatment to various additional disorders of cognitive function, including but not limited to, vascular dementia, Down syndrome, traumatic brain injury and mild cognitive impairment.
As noted above, reduced levels of neurotransmitters including acetylcholine have been reported in dementias of the Alzheimer type and related disorders. In particular, a deficit in acetylcholine-mediated transmission is thought to contribute to the cognitive and certain of the neurobehavioral abnormalities associated with these disorders. Accordingly, drugs known to augment cholinergic transmission in the CNS are widely used in therapy.
AChEIs are now part of the standard care for patients suffering from a dementia of the Alzheimer type and are widely used off label for various other chronic progressive disorders of cognitive function. AChEIs have the enhancement of acetylcholine-mediated neurotransmission as a general mechanism of action. All act in the human CNS to increase and prolong the availability of acetylcholine by inhibiting its degradatory enzyme acetylcholinesterase. Four AChEIs have been approved by the U.S. FDA for the treatment of Alzheimer's disease and for Parkinson's disease dementia: tacrine, donepezil [Aricept®], rivastigmine [Exelon®] and galantamine [Razadyne®]. AChEIs are available in various formulations including immediate release forms such as tablets, capsules and solutions as well as rapid dissolving and extended release forms for oral administration as well as those for parenteral (e.g. transdermal) administration.
Other AChEIs, in particular tacrine analogs, such as ipidacrine; phenserine and their analogs; icopezil; and zanapezil are under evaluation.
Augmentation of cholinergic transmission in the CNS by currently available AChEIs confers therapeutic benefit to patients with Alzheimer type dementias. Therapeutic efficacy can be measured by the degree of improvement in cognitive dysfunction and other neurobehavioral abnormalities associated with these disorders using standardized scales.
Unfortunately, however, none of the currently available cholinomimetic or medications offer more than modest clinical benefit for some patients suffering from any of the aforementioned dementing disorders, even when these medications are administered at their maximum safe and tolerated doses. This is the first problem limiting the success of current AChEI therapy of Alzheimer type dementias.
Carefully conducted clinical trials of donepezil (Rogers et al., Neurology 1998, 50, 136-45; Winblad et al. Neurology. 2001 August 14; 57(3):489-95), rivastigmine (Rösler, et al., Brit. Med. J. 1999, 318, 633-38; Farlow et al. Eur. Neurol., 2000, 44, 236-41) and galantamine (Raskind et al., Neurology, 2000, 54, 2261-68; Tariot et al., Neurology, 2000, 54, 2269-76) in patients with dementias of the Alzheimer type demonstrated small, but statistically significant, benefits on cognitive and global measures relevant to dementia. The magnitude of the effect in pivotal clinical trials was on the order of a 2.8 point improvement on the 70-point cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog), or 1-1.5 point improvement on the 30-point Mini-Mental Status Examination (MMSE) compared to placebo over six months. Differences in global measures assessed by the 7-point Clinician Interview-Based Impression of Change scale (CIBIC) were on the order of 0.3-0.5 points in patients receiving an AChEI compared to those receiving placebo. Efficacy was similar for the three commonly used AChEIs. AChEIs also appear to have a beneficial effect on the behavioral and neuropsychiatric symptoms in patients with Alzheimer type dementias.
A second problem limiting the success of current AChEI therapy of Alzheimer type dementias is that, even at recommended amounts, all these drugs produce dose limiting adverse reactions. These side effects commonly include, for the aforementioned AChEIs tacrine, donepezil, rivastigmine and galantamine: anorexia, nausea, vomiting, diarrhea, abdominal pain and weight loss (Physicians Desk Reference 2008, Thomson PDR, Montvale, N.J.).
The most frequently reported adverse effects of rivastigmine, for example, are gastrointestinal, especially nausea. About half of patients who take this drug in the recommended therapeutic oral dose range of 6-12 mg/day become nauseated and about one-third vomit at least once. Vomiting was severe in 2% of rivastigmine-treated patients and was mild or moderate in 14%. Five percent of patients discontinued rivastigmine because of vomiting, compared to less than 1% on placebo. A loss of appetite was reported by 17% of patients, and weight declined in 25% during rivastigmine therapy (averaging 7 to 10 pounds). Presumably, the drug-induced anorexia, nausea and vomiting contribute to the observed weight loss. These untoward gastrointestinal effects, as well as others occurring with AChEI treatment, make it difficult to increase rivastigmine dosage above 6 mg daily in most patients.
Adverse events significantly reduce the safety and tolerability of AChEI therapy. Attempts to limit them in clinical practice now rely on initiating treatment with a low dose and then escalating the dose slowly. Nevertheless, in current clinical practice, AChEI dosage is guided mainly by side effects and not by therapeutic effects in contrast to most drugs used in the treatment of neuropsychiatric disease. The administration of higher doses than recommended doses tends to increase the frequency and severity of these side effects as well as introduce additional kinds of adverse reactions. These include those generally found with high dose administration of cholinomimetics. In view of the frequency and potential severity of these high dose adverse effects, maximum recommended oral doses of AChEIs are rarely intentionally exceeded in clinical practice.
It is reported that recommended maximal dose levels of these drugs typically achieve only about 45% acetyl cholinesterase inhibition in the CNS of Alzheimer disease patients (Brannan S et al. ACNP 46th Annual Meeting, Program No. 4. Boca Raton Fla., Dec. 10, 2007—“Brannan 2007”) and that inhibition of acetyl cholinesterase activity and cognitive improvement are significantly correlated (Giacobini et al. J Neural Transm. 2002 July; 109(7-8):1053-65) and that, ordinarily, a higher degree of enzyme blockade must be attained for maximum functional effect (Jann et al., Clin Pharmacokinet. 2002; 41(10):719-39—“Jann 2002”).
On the other hand, doubling the dose of rivastigmine, which became clinically practical when AChEI administration by immediate release tablets was replaced by skin patches, which diminished side effects by blunting peak blood levels, significantly increased the amount of cognitive improvement in patients with Alzheimer's disease without increasing side effects.
By virtue of being dose limiting, these adverse effects also constrain the efficacy of AChEI therapy. Studies in animal models of human cognitive dysfunction indicate a direct dose-response relation between the amount of acetyl choline esterase inhibition and the degree of cognitive improvement (Bennett B M et al., Neuropsychopharmacology. 2007 March; 32(3):505-13). Similar conclusions have been drawn regarding AChEI effects on cognitive and behavioral symptoms in human patients with Alzheimer's disease (Jann 2002; Winblad B, Cummings J, Andreasen N, Grossberg G, Onofrj M, Sadowsky C, Zechner S, Nagel J, Lane R. Int J Geriatr Psychiatry. 2007 May; 22(5):456-67).
The precise causes of the vomiting and related gastrointestinal symptoms induced by AChEI therapy are not known. Presumably, they reflect the cholinergic receptor hyperstimulation attending AChEI administration. Vomiting is coordinated in a center located at the base of the brain. The vomiting center communicates with the nearby chemoreceptor trigger zone, whose stimulation can lead to such complaints of gastrointestinal distress as anorexia, nausea and vomiting. The chemoreceptor trigger zone, which contains numerous serotonin 5-HT3 and dopamine D2 receptors as well as those for acetylcholine, opioids and substance P, lies outside the blood-brain barrier. Systemically administered antiemetics acting at these or related transmitter receptor sites can thus inhibit vomiting even if unable to enter the CNS.